If one doesn't develop symptoms of ebola 56 days after potential exposure, is the threat of the virus zilch?

Low; not zilch. As: always, the route & severity of exposure into actual bodily fluids are key issues as to whether infection occurs or not along with speed of onset symptoms. There are different variants with different immunogenic characteristics, cell & tissue susceptibility issues. Thus, while time sounds very good, reality does not always lend itself to simple absolute certainties. Good luck & stay vigilant.